This invention relates in general to plasma arc cutting and welding processes and apparatus. More specifically, it relates to a process and apparatus for reducing electrode wear.
Plasma arc torches have a wide variety of applications such as the cutting of thick plates of steel and the cutting of comparatively thin sheets of galvanized metal commonly used in heating, ventilating and air conditioning (HVAC) systems. The basic components of a plasma arc torch include a torch body, an electrode (cathode) mounted within the body, a nozzle (anode) with a central exit orifice, a flow of an ionizable gas, electrical connections, passages for cooling, and arc control fluids, and a power supply that produces a pilot arc in the gas, typically between the electrode and the nozzle, and then a plasma arc, a conductive flow of the ionized gas from the electrode to a workpiece. The gas can be non reactive, e.g. nitrogen, or reactive, e.g. oxygen or air.
Various plasma arc torches of this general type are described in U.S. Pat. Nos. 3,641,304 to Couch and Dean, 3,833,787 to Couch, 4,203,022 to Couch and Bailey, 4,421,970 to Couch, 4,791,268 to Sanders and Couch, 4,816,637 to Sanders and Couch, and 4,861,962 to Sanders and Couch, all commonly assigned with the present application. Plasma arc torches and related products are sold in a variety of models by Hypertherm, Inc. of Hanover, New Hampshire. The MAX 100 brand torch of Hypertherm is typical of the medium power torches (100 ampere output) using air as the working gas and useful for both plate fabrication and HVAC applications.
In all plasma arc torches, a common and heretofore unsolved problem has been a substantial wear of the electrode, particularly when the electrode is used with reactive gases such as oxygen or air. (Improved wear, other conditions being the same, is observed when using non reactive gases such as nitrogen or argon as the plasma gas, but the performance using pure oxygen is superior at least when used to cut certain materials such as mild steel. Similarly, air is superior to pure oxygen with respect to wear, but there is again a performance trade off.) As an example of this wear problem, the standard electrode for the MAX 100 brand torch of Hypertherm, Inc. shows wear as a generally concave pit on the lower end of the electrode, or more precisely, on an emitting element of hafnium mounted on the electrode. On average a wear depth of about 0.025 inch is observed in such a Hypertherm brand electrode after 120 cut cycles operating with oxygen or air. The wear results of commercially available units of others, as measured by Hypertherm, Inc., are typically worse. For the MAX 100 brand torch, when the wear produces a pit depth of 0.060 inch or more, Hypertherm, Inc. recommends that the electrode be replaced. In ordinary use, the electrode of a plasma arc cutting torch operating with reactive gases typically requires replacement after 0.5 to 2 hours of use depending strongly on the number of on off cycles. Wear considerations are significant not only because they necessitate the repeated replacement of a component, but also because they limit the maximum power that can be applied to a given torch.
In plasma arc cutting, it is also important to note that the quality of the cut is highly dependent on the flow pattern of the gas in a plasma chamber, defined at least in part as the region between the electrode and the nozzle. In particular, a swirling flow produced by injecting the gas tangentially into the plasma chamber has been found to be essential to produce a high quality cut. A swirling gas flow pattern is also important in stabilizing the plasma arc so that it exits the torch to attach to and cut the workpiece, but does not contact the torch nozzle itself. The nozzle is the principal component that is damaged by the arc when the arc is not well controlled.
Another design consideration is the very high temperatures of the plasma, e.g. greater than 10,000.degree. C. These temperatures introduce corresponding changes in the gas properties such as its density and viscosity. These considerations are significant on start up and cut off. On start up the arc rapidly heats the gas which significantly decreases the gas density exiting the nozzle orifice. This presents the situation where the gas flow is choked in the nozzle orifice region. On cut off, the situation reverses and there is a tendency for the gas in the plasma chamber to blow out of the chamber very suddenly as the mass flow increases when the temperature drops.
It is common industry practice to use hafnium or zirconium as the cathodic emitter insert in the electrode. Hafnium, as of today, is the best choice for the cathodic emitting element when cutting with a reactive gas plasma. It exhibits the least wear of all other materials tried for this application, but is more costly than other materials. These electrodes nevertheless require frequent replacement. Lower wear has been associated with lower current levels, but at some point the reduction in performance associated with a reduced operating current becomes too great. Cooling the electrode has also been used to increase electrode life, whether by way of a gas flow or water flow placed in good thermal communication with the electrode. However, water cooling is expensive, cumbersome and is not desirable for low current units, e.g. those rated below 100 amperes. Air cooling is less efficient and limits the maximum operating current of the torch, even one carrying a comparatively low current. Therefore, to date, the only practical solution to the electrode wear problem has been to replace the entire electrode again and again, despite the clear economic disadvantages of this approach.
It is therefore a principal object of the present invention to reduce the wear on the electrode of a plasma arc torch significantly and thereby extend its life.
Another principal object of this invention is to reduce electrode wear and thereby allow operation at higher current levels than are presently feasible, even when operating with reactive gases.
Another object of the invention is to achieve a better cut quality than has heretofore been possible by allowing a greater level of swirl.
Another object of the invention is to provide the foregoing advantages while using standard electrode and nozzle constructions and without any significant increase in the incidence of damage to torch parts such as nozzle gouging.
Yet another object of the invention is to provide the foregoing advantages for existing plasma arc torch systems using only comparatively simple and inexpensive modifications.
A still further object is to provide the foregoing advantages at a favorable cost of manufacture and operation.